Switching power supply and electronic apparatus using the same

ABSTRACT

A switching power supply includes a control circuit disposed between a feedback winding and a switching device. The control circuit includes an on-period control circuit that, in an operation under a non-low load condition, controls an on-period of the switching device such that the on-period decreases with decreasing load, a minimum on-period setting circuit that disables the on-period control circuit in an operation under a low load condition so that the on-period of the switching device does not become shorter than a predetermined minimum on-period, and an off-period control circuit that controls an off-period of the switching device in the operation under the low load condition such that the off-period increases with decreasing load. As a result, the output voltage is maintained at a constant value in accordance with a feedback signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a switching power supply and anelectronic apparatus including a switching power supply, and moreparticularly, to a switching power supply that operates with low losseven under a low load condition, and an electronic apparatus including aswitching power supply.

[0003] 2. Description of the Related Art

[0004] In switching power supplies such as an RCC switching powersupply, the switching frequency changes depending upon whether a load islow or high. When the load is high, the on-period and the off-period areboth increased, and thus the switching frequency decreases. On the otherhand, when the load is low, the on-period and the off-period are bothdecreased, and thus, the switching frequency increases. Various types ofloss occur in the switching power supply. One type of loss is aswitching loss which occurs each time a switching device switches.Because the switching loss occurs each time the switching deviceswitches, if the frequency increases with decreasing load, a largeswitching loss occurs. Thus, the switching loss is a major component ofthe loss of the switching power supply when the load is within a lowrange. When the load is very low, intermittent oscillation may occurs,which causes an increase in output ripple or noise that is generated.

[0005] Japanese Unexamined Patent Application Publication No. 2001-16849discloses a technique for reducing the increase in the switchingfrequency during the operation under a low load condition, by reducingthe switching rate during the operation under the low load condition. Inthis technique, a minimum on-period of a switching device is set, and,when the output voltage increases as the load decreases, a signal whichcontrols the turning-on of the switching device is blocked so as toprevent the switching device from turning on, thereby preventing theon-period of the switching device from decreasing to less than theminimum on-period, whereby the switching rate during the operation underthe low load condition is reduced to achieve a reduction in switchingloss.

[0006] However, in the switching power supply disclosed in JapaneseUnexamined Patent Application Publication No. 2001-16849, the signalwhich turns the switching device on is blocked to prevent the turning-onof the switching device only when the output voltage is greater than aset value, and thus, the off-period of the switching device and theswitching frequency are not constant even under a static load condition.Therefore, the problems associated with the increase in the outputripple and noise are not adequately solved.

SUMMARY OF THE INVENTION

[0007] In order to overcome the above-described problems, preferredembodiments of the present invention provide a switching power supplyhaving low switching loss without producing problems associated withintermittent oscillation or an increase in output ripple duringoperation under a low load condition, and an electronic apparatusincluding such a novel switching power supply.

[0008] According to a preferred embodiment of the present invention, aswitching power supply includes a transformer having a primary winding,a secondary winding and a feedback winding, a switching device connectedin series to the primary winding, a control circuit connected betweenthe feedback winding and a control terminal of the switching device, arectifying and smoothing circuit connected to the secondary winding torectify and smooth a voltage generated in the secondary winding andoutputting a resultant rectified and smoothed voltage, and an outputvoltage detecting circuit to detect the voltage output from therectifying and smoothing circuit and output a feedback signal to thecontrol circuit, wherein the control circuit controls the output voltageat a constant value in accordance with the feedback signal by, in anoperation under a non-low load condition, controlling the on-periodwithin a range that is greater than a desired minimum on-period, while,in an operation under a low load condition, setting the on-period to theminimum on-period and controlling the off-period.

[0009] According to another preferred embodiment of the presentinvention, a switching power supply includes a transformer having aprimary winding, a secondary winding and a feedback winding, a switchingdevice connected in series to the primary winding, a control circuitconnected between the feedback winding and a control terminal of theswitching device, a rectifying and smoothing circuit connected to thesecondary winding to rectify and smooth a voltage generated in thesecondary winding and outputting a resultant rectified and smoothedvoltage, and an output voltage detecting circuit to detect the voltageoutput from the rectifying and smoothing circuit and output a feedbacksignal to the control circuit, wherein the control circuit includes anon-period control circuit that controls an on-period of the switchingdevice during an operation under a non-low load condition such that theon-period decreases with decreasing load, a minimum on-period settingcircuit that disables the on-period control circuit from turning on theswitching device during an operation under a low load condition suchthat the on-period of the switching device does not become shorter thana desired minimum on-period, and an off-period control circuit thatcontrols an off-period of the switching device during the operationunder the low load condition such that, when the operation of theon-period control circuit is disabled by the minimum on-period settingcircuit, the off-period increases with decreasing load, whereby theoutput voltage is controlled at a constant value in accordance with thefeedback signal.

[0010] The switching power supply according to preferred embodiments ofthe present invention operates in a current-critical mode during theoperation under the non-low load condition.

[0011] In the switching power supply according to preferred embodimentsof the present invention, the on-period control circuit includes a firstcapacitor that is charged or discharged during the on-period of theswitching device, and the timing of turning-off of the switching deviceis determined by a time at which the voltage across the first capacitorreaches or crosses a voltage determined by the feedback signal, theminimum on-period setting circuit includes a second capacitor that ischarged or discharged during the on-period of the switching device, theturning-off of the switching device by the on-period control circuit isdisabled until the voltage across the second capacitor reaches orcrosses a reference voltage, and the off-period control circuit includesa third capacitor that is charged or discharged during the off-period ofthe switching device, and the timing of turning-on of the switchingdevice is determined by a time at which the voltage across the thirdcapacitor reaches or crosses a voltage determined by the feedbacksignal.

[0012] In the switching power supply according to preferred embodimentsof the present invention, the first capacitor also functions as thethird capacitor, or as the second and third capacitors.

[0013] In the switching power supply according to preferred embodimentsof the present invention, when the on-period control circuit isoperating, the timing of turning-off of the switching device isdetermined at a time at which the voltage across the first capacitorreaches or crosses, in one direction, the voltage determined by thefeedback signal, and when the operation of the on-period control circuitis disabled by the minimum on-period setting circuit, the timing ofturning-on of the switching device is determined at a time at which thevoltage across the first capacitor reaches or crosses, in a reversedirection, the voltage determined by the feedback signal.

[0014] In the switching power supply according to preferred embodimentsof the present invention, the minimum on-period setting circuit is adevice that is included in the on-period control circuit and disablesthe on-period control circuit from turning off the switching device fora fixed period after the switching device turns on, during the operationunder low load condition.

[0015] In the switching power supply according to preferred embodimentsof the present invention, the minimum on-period setting circuitdischarges the first capacitor when the switching device turns on, andthe minimum on-period setting circuit disables the switching device fromturning off thereafter until the voltage of the first capacitor ischarged thereafter until it reaches or crosses a predetermined voltage.

[0016] According to another preferred embodiment of the presentinvention, an electronic apparatus including the switching power supplyaccording to preferred embodiments described above is provided.

[0017] In the switching power supply according to preferred embodimentsof the present invention, as described above, the switching loss duringoperation under a low load condition is greatly reduced. Furthermore,intermittent oscillation is prevented and ripples are greatly reduced.

[0018] In the electronic apparatus according to the present invention,the efficiency in the standby state is greatly improved.

[0019] Other features, elements, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments with reference to the drawingsthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a circuit diagram of a switching power supply accordingto a preferred embodiment of the present invention.

[0021]FIG. 2A is a diagram showing temporal changes, under a rated loadcondition, in Vbias, Vc4, Vc5, and Vc6 in the switching power supplyshown in FIG. 1, and FIG. 2B is a diagram showing similar temporalchanges under a low load condition.

[0022]FIG. 3 is a circuit diagram of a switching power supply accordingto another preferred embodiment of the present invention.

[0023]FIG. 4A is a diagram showing temporal changes, under the ratedload condition, in Vbias, Vc6, and Vc7 in the switching power supplyshown in FIG. 3, and FIG. 3B is a diagram showing similar temporalchanges under the low load condition.

[0024]FIG. 5 is a circuit diagram of a switching power supply accordingto still another preferred embodiment of the present invention.

[0025]FIG. 6A is a diagram showing temporal changes, under the ratedload condition, in Vbias and Vc7 in the switching power supply shown inFIG. 5, and FIG. 6B is a diagram showing similar temporal changes underthe low load condition.

[0026]FIG. 7 is a perspective view of an electronic apparatus accordingto another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0027]FIG. 1 is a circuit diagram of a switching power supply accordingto a preferred embodiment of the present invention. As shown in FIG. 1,the switching power supply 1 includes a transformer T including aprimary winding N1, a secondary winding N2, and a feedback winding N3, aDC power supply Vin and a MOSFET functioning as a switching device Q1,both connected in series to the primary winding N1, a rectifying andsmoothing circuit 2 connected to the secondary winding N2, an outputvoltage detecting circuit 3 connected to the rectifying and smoothingcircuit 2, and a control circuit 4 provided between the feedback windingN3 and a gate serving as a control terminal of the switching device Q1.The output voltage detecting circuit 3 includes a light emitting diodePD defining one element of a photocoupler for outputting a feedbacksignal to the control circuit 4, wherein the photocoupler is connectedsuch that when the output voltage increases due to a reduction in load,the intensity of emitted light increases.

[0028] The control circuit 4 is described below. One end of the feedbackwinding N3 is connected to the gate of the switching device Q1 via acapacitor C1 and a path between the emitter and the collector of atransistor Q2, and the other end of the feedback winding N3 is connectedto the source of the switching device Q1 and thus connected to ground. Adiode D1 is connected between the emitter and collector of thetransistor Q2, and a capacitor C2 is connected between the emitter andthe base of the transistor Q2. Furthermore, the emitter of thetransistor Q2 is connected to the DC power supply Vin via a startingresistor R1, and the base thereof is grounded via a resistor R2 and apath between the collector and the emitter of a transistor Q3.

[0029] A phototransistor PT coupled with the light emitting diode PD inthe output voltage detecting circuit 3 is arranged such that thecollector is connected to a constant current source I, the emitter isconnected to ground, a capacitor C3 is connected between the collectorand emitter, and the collector is connected to an inverting inputterminal of a comparator IC1 and also to a non-inverting input terminalof a comparator IC2. The constant current source I is produced using theDC power supply Vin or a power supply obtained by rectifying andsmoothing a voltage of the feedback winding N3.

[0030] One end of the feedback winding N3 is connected to ground via aresistor R3 and a capacitor C4 defining the first capacitor, wherein thenode between the resistor R3 and the capacitor C4 is connected to thenon-inverting input terminal of the comparator IC1. The output of thecomparator IC1 is connected to one input of an AND circuit IC4. The oneend of the feedback winding N3 is also connected to ground via aresistor R4 and to a parallel circuit of a Zener diode D3 and acapacitor C5 functioning as the third capacitor, wherein the nodebetween the resistor R4 and the capacitor C5 is connected to theinverting input terminal of the comparator IC2. The output of thecomparator IC2 is connected to the base of the transistor Q3 via aresistor R5.

[0031] Furthermore, the one end of the feedback winding N3 is alsoconnected to ground via a resistor R6 and a parallel circuit of a diodeD2 and a capacitor C6 functioning as the second capacitor, wherein thenode between the resistor R6 and the capacitor C6 is connected to thenon-inverting terminal of a comparator IC3. The inverting input terminalof the comparator IC3 is connected to a reference voltage source Vref,and the output of the comparator IC3 is connected to the other input ofthe AND circuit IC4. The output of the AND circuit 14 is connected tothe base of a transistor Q4 via a resistor R7. The gate of the switchingdevice Q1 is connected to ground via a path between the collector andthe emitter of the transistor Q4.

[0032] The operation of the switching power supply 1 is described belowwith reference to FIG. 2. FIG. 2 shows changes, with time, in thevoltage Vbias of the feedback winding N3, the voltage Vc4 across thecapacitor C4 defining the first capacitor, the voltage Vc5 across thecapacitor C5 serving as the third capacitor, and the voltage Vc6 acrossthe capacitor C6 defining the second capacitor, for (a) operation underthe rated load condition, and (b) operation under the low loadcondition. In FIG. 2, Vfb denotes the collector voltage of thephototransistor PT, which is input, as a feedback voltage, to theinverting input terminal of the comparator IC1. Vfb changes with load.However, when the output voltage does not change, Vfb is maintained at asubstantially constant value. Herein, a circuit defined by the constantcurrent source I, the phototransistor PT, and the capacitor C3 isreferred to as a feedback voltage generator. Vz denotes a cathodevoltage of the Zener diode D3 in a breakdown state, which is applied tothe inverting input terminal of the comparator IC2. Vref denotes thevoltage of the reference voltage source Vref, which is connected to theinverting input terminal of the comparator IC3.

[0033] The operation under the rated-load condition, which is aparticular type of non-low load condition, is described below forvarious operation stages. Herein, the low load condition refers to acondition where the load is lower than a predetermined value, and thenon-low load condition refers to a state where the load is higher thanthe predetermined value, wherein the operation with the rated loadoccurs under the non-low load condition.

[0034] (t=0 to t1)

[0035] When the switching device Q1 turns on, a current flows throughthe primary winding N1, and the voltage Vbias of the feedback winding N3becomes positive. As a result, charging of the capacitors C4, C5, and C6begins. The charging of the capacitor C5 stops when the voltage Vc5across the capacitor C5 reaches Vz, and no further changing occurs.Under the rated load condition, Vfb has a higher level than Vz, andthus, the output of the comparator IC2 is at an H-level (high level).When the output of the comparator IC2 is at the H-level, the transistorQ3 is in an on-state, and thus, the transistor Q2 is also in theon-state.

[0036] (t=t1 to t2)

[0037] When the voltage Vc6 across the capacitor C6 becomes greater thanVref, the output of the comparator IC3 goes to the H-level. However, atthis point of time, the voltage Vc4 across the capacitor C4 has not yetexceeded Vfb, and thus the output of the comparator IC1 remains at anL-level (low level) and the output of the AND circuit IC4 also remainsat the L-level.

[0038] (t=t2 to t3)

[0039] When the voltage Vc4 across the capacitor C4 becomes greater thanVfb, the output of the comparator IC1 goes to the H-level. As a result,the two inputs of the AND circuit IC4 both become high and the outputthereof also becomes also high. This transition to the H-level turns thetransistor Q4 on via the resistor R7. The turning on of the transistorQ4 causes the switching device Q1 to turn off. Thus, the on-period ofthe switching device Q1 ends. That is, the crossing of Vfb determined bythe feedback signal, in the increase in the voltage Vc4 across thecapacitor C4 function the first capacitor, functions as a trigger thatdetermines the timing of the turning-off of the switching device Q1, andthus, determines the on-period of the switching device Q1.

[0040] Because the resistance between the collector and the emitter ofthe phototransistor PT decreases as the load reduces and thus, as theintensity of light received from the light emitting diode PD increases,Vfb decreases as the load decreases. Therefore, the time required forthe voltage Vc4 across the capacitor C4 to exceed Vfb decreases as theload decreases. Thus, a circuit including the resistor R3, the capacitorC4, the comparator IC1, the AND circuit IC4, the resistor R7, thetransistor Q4, and the feedback voltage generator defines an on-periodcontrol circuit that controls the on-period of the switching device Q1such that the on-period decreases as the load decreases in operationunder the non-low load condition.

[0041] When the switching device Q1 turns off, a current begins to flowfrom the secondary winding N2 into the rectifying and smoothing circuit2, and the voltage Vbias of the feedback winding N3 becomes negative. Asa result, discharging of the capacitors C4, C5, and C6 begins. If thedischarge of the capacitor C4 begins, the voltage Vc4 across thecapacitor C4 immediately decreases to less than Vfb, and thus, theoutput of the comparator IC1 goes to the L-level and the output of theAND circuit IC4 goes to the L-level. As a result, the transistor Q4returns to the off-state. That is, in the above-described process, thetransistor Q4 turns on only temporarily in order to turn off theswitching device Q1.

[0042] As understood from the above description, because the voltage Vc4across the capacitor C4 very quickly returns to a level lower than Vfbafter becoming higher than Vfb, the turning-off of the switching deviceQ1 occurs when the voltage Vc4 reaches Vfb without exceeding it.

[0043] (t=t3 to t4)

[0044] When the switching device Q1 is in the off-state, if the currentflowing out of the secondary winding N2 into the rectifying andsmoothing circuit 2 becomes zero, the voltage Vbias of the feedbackwinding N3 starts to resonate.

[0045] (t≧t4)

[0046] The first positive portion of a resonating wave of the voltageVbias is applied to the gate of the switching device Q1 via thetransistor Q2 in the on-state, and thus, the switching device Q1 turnson. That is, the transition-to-zero of the current from the secondarywinding N2 to the rectifying and smoothing circuit 2 serves as a triggerthat turns on the switching device Q1. If the switching device Q1 turnson, the resonance of the voltage Vbias stops, and the voltage Vbiasagain becomes positive as at t=0. Thereafter, the process after t=0 isrepeated.

[0047] In the operation under the rated load condition (under non-lowload condition), as described above, when the current flowing throughthe primary winding N1 stops, a current immediately begins to flow fromthe secondary winding N2 to the rectifying and smoothing circuit 2, and,if the current flowing out of the secondary winding N2 stops, theswitching device Q1 immediately turns on and a current starts to flowthrough the primary winding N1. Such an operation mode is referred to asa current-critical mode. That is, under the rated load condition, theswitching power supply 1 operates in the current-critical mode.

[0048] When the on-period of the switching device Q1 is controlled inthe operation under the non-low load condition, magnetic energy storedin the transformer T changes in response to a change in the on-periodresulting from a change in the load, and thus, a corresponding changeoccurs in the off-period during which the magnetic energy is released.

[0049] The operation under the low load condition is now described belowfor various operation stages.

[0050] (t=0 to t1)

[0051] When the switching device Q1 turns on, a current flows throughthe primary winding N1, and the voltage Vbias of the feedback winding N3becomes positive. As a result, charging of the capacitors C4, C5, and C6begins. When the switching device Q1 turns on, as will be describedlater, the capacitor C6 is in a completely discharged state, and thus,the voltage Vc6 across the capacitor C6 is equal to 0 V. The charging ofthe capacitor C5 stops when the voltage Vc5 across the capacitor C5 hasreached Vz, and no further changing occurs. Under the low loadcondition, Vfb is less than Vz, and thus, the output of the comparatorIC2 is at the L-level. When the output of the comparator IC2 is at theL-level, the transistor Q3 is in the off-state, and thus, the transistorQ2 is also in the off-state.

[0052] (t=t1 to t2)

[0053] When the voltage Vc4 across the capacitor C4 becomes greater thanVfb, the output of the comparator IC1 goes to the H-level, and theon-period control circuit attempts to begin operating. However, at thispoint of time, the voltage Vc6 across the capacitor C6 has not yetreached Vref, and thus, the output of the comparator IC3 still remainsat the L-level and the output of the AND circuit IC4 also remains at theL level. That is, crossing the voltage Vfb during the increase in thevoltage Vc4 of the capacitor C4 defining the first capacitor does notfunction as a trigger that causes the switching device Q1 to turn off,and the operation of the on-period control circuit is disabled.

[0054] (t=t2 to t3)

[0055] When the voltage Vc6 across the capacitor C6 becomes greater thanVref, the output of the comparator IC3 goes to the H-level. As a result,the levels of the two inputs of the AND circuit IC4 both become high andthe output of the AND circuit IC4 also becomes high, which causes thetransistor Q4 to turn on. The turning-on of the transistor Q4 causes theswitching device Q1 to turn off. Thus, the on-period of the switchingdevice Q1 terminates. That is, crossing the voltage Vref in the increasein the voltage Vc6 of the capacitor C6 defining the second capacitorfunctions as a trigger that determines the timing of turning off theswitching device Q1, and thus, determines the on-period. In other words,the minimum on-period of the switching device Q1 is determined by aperiod required for the voltage Vc6 across the capacitor C6 to exceedVref after increasing from 0 V. Thus, a circuit including the resistorR6, the capacitor C6, the diode D2, the reference voltage source Vref,and the comparator IC3 defines a minimum on-period setting circuit thatsuppresses the turning-off operation of the switching device Q1.

[0056] When the switching device Q1 turns off, a current starts to flowfrom the secondary winding N2 into the rectifying and smoothing circuit2, and the voltage Vbias of the feedback winding N3 becomes negative. Asa result, discharging of the capacitors C4, C5, and C6 begins. If thedischarging of the capacitor C6 begins, the voltage Vc6 across thecapacitor C6 immediately becomes less than Vref, and thus, the output ofthe comparator IC3 goes to the L-level and the output of the AND circuitIC4 goes to the L-level. As a result, the transistor Q4 returns to theoff-state. That is, in the above-described process, the transistor Q4turns on only temporarily in order to turn off the switching device Q1.The diode D2 prevents the capacitor C6 from being charged in an oppositedirection.

[0057] (t=t3 to t4)

[0058] When the switching device Q1 is in the off-state, if the currentflowing out of the secondary winding N2 into the rectifying andsmoothing circuit 2 becomes zero, the voltage Vbias of the feedbackwinding N3 starts to resonate. At this point of time, the transistor Q2is in the off-state, and thus, the resonating voltage Vbias is notapplied to the gate of the switching device Q1 and the resonatingvoltage Vbias attenuates with time. That is, the turning-on of theswitching device Q1 by the resonating voltage Vbias is prevented.Thereafter, therefore, no current flows through the primary winding N1and the secondary winding N2. As a result, the current-critical mode isaborted. On the other hand, the capacitor C5 is further discharged andthe voltage Vc5 across the capacitor C5 further decreases. Thecapacitors C4 and C6 are also further discharged, and the voltages Vc4and Vc6 approach 0 V with the passage of time.

[0059] (t≧t4)

[0060] If the voltage Vc5 across the capacitor C5 decreases until itbecomes less than Vfb, the output of the comparator IC2 goes to theH-level. As a result, the transistor Q3 turns on, and thus, thetransistor Q2 turns on. This causes the charge, which has been stored inthe capacitor C1 by t=t4, to be applied to the gate of the switchingdevice Q1 via the transistor Q2. As a result, the switching device Q1turns on. Thus, crossing the voltage Vfb during the change in thevoltage Vc5 across the capacitor C5 defining the third capacitorfunctions as a trigger that determines the timing of the turning-on ofthe switching device Q1. Because Vfb decreases with decreasing load, thetime required for the voltage Vc5 across the capacitor C5 to decreaseuntil it becomes less than Vfb increases with decreasing load. Thus, acircuit including the resistor R4, the capacitor C5, the Zener diode D3,the comparator IC2, the resistor R5, the transistor Q3, the resistor R2,the capacitor C2, and the feedback voltage generator defines anoff-period control circuit that controls the off-period of the switchingdevice Q1 in the operation under low load condition such that theoff-period increases with decreasing load. Note that the capacitance ofthe capacitor C6 and the resistance of the resistor R6 are set such thatthe capacitor C6 is completely discharged by the above-described pointof time. Thereafter, the process after t=0 is repeated.

[0061] In the switching power supply 1, as described above withreference to FIGS. 1 and 2, the on-period control circuit controls theon-period of the switching device in the operation under the non-lowload condition such that the output voltage is maintained at a constantvalue, while, in the operation under low load condition, the minimumon-period setting circuit sets the on-period of the switching device tothe minimum on-period, and the off-period control circuit controls theoff-period such that the output voltage is maintained at the constantvalue.

[0062] Thus, an increase in the switching frequency in the operationunder the low load condition is suppressed and the switching frequencywith decreasing load is reduced, thereby achieving a reduction inswitching loss in the operation under the low load condition.Furthermore, because the off-period of the switching device iscontrolled continuously depending on the load in the low-load operation,intermittent oscillation is prevented, and thus, output ripples aregreatly decreased. Furthermore, because the duty ratio of the switchingoperation of the switching device is continuous at the boundary betweenthe low load mode and the non-low load mode, the switching operation isprevented from becoming discontinuous when the load is close to theabove-described boundary.

[0063]FIG. 3 is a circuit diagram of a switching power supply accordingto another preferred embodiment of the present invention. In FIG. 3,similar components to those in FIG. 1 are denoted by similar referencenumerals, and the description thereof is omitted.

[0064] As shown in FIG. 3, the switching power supply 10 includes acontrol circuit 11 provided between the feedback winding N3 and the gateof a switching device Q1, wherein a main portion of the control circuit11 is defined by an integrated circuit 12. The control circuit 11including constituent elements of the integrated circuit 12 is describedbelow.

[0065] One end of the feedback winding N3 is connected to the integratedcircuit 12 via a rectifying and smoothing circuit including a diode D4and a capacitor C8. The voltage output from the rectifying and smoothingcircuit is applied to each element of the integrated circuit 12. Thenode between the diode D4 and the capacitor C8 is connected to a DCpower supply Vin via a starting resistor R1.

[0066] A phototransistor PT coupled with the light emitting diode PD inthe output voltage detecting circuit 3 is arranged such that thecollector is connected to a constant current source I, the emitter isconnected to ground, a capacitor C3 is connected between the collectorand emitter, and the collector is connected to a non-inverting inputterminal of a comparator IC6 and also to an inverting input terminal ofa comparator IC8. The constant current source I is provided using thevoltage supplied to the integrated circuit 12 from the rectifying andsmoothing circuit including the diode D4 and the capacitor C8.

[0067] The one end of the feedback winding N3 is connected to anon-inverting input terminal of a comparator IC5. An inverting inputterminal of the comparator IC5 is connected to an offset voltage sourceVoff having a small negative voltage such that the output of thecomparator IC5 becomes high when the voltage of the non-inverting inputterminal is equal to zero.

[0068] The one end of the feedback winding N3 is also connected toground via a resistor R8 and a capacitor C7, and the node between theresistor R8 and the capacitor C7 is connected to the inverting inputterminal of the comparator IC6 and also to the non-inverting inputterminal of the comparator IC8. The capacitor C7 defines the first andthird capacitors.

[0069] The one end of the feedback winding N3 is also connected toground via a resistor R6 and a parallel circuit of a diode D2 and acapacitor C6 defines the second capacitor. The node between the resistorR6 and the capacitor C6 is connected to a non-inverting input terminalof a comparator IC9. An inverting input terminal of the comparator IC9is connected to a reference voltage source Vref, and the output of thecomparator IC9 is connected to one input of an AND circuit IC10.

[0070] The outputs of the comparators IC5 and IC6 are connected to twoinputs of an AND circuit IC7, and the output of the AND circuit IC7 isconnected to a set terminal S of an RS flip-flop IC11. Similarly, theoutputs of the comparators IC8 and IC9 are connected to two inputs of anAND circuit IC10, and the output of the AND circuit IC10 is connected toa reset terminal R of an RS flip-flop IC11. The output terminal Q of theRS flip-flop IC11 is connected to the gate of the switching device Q1via the drive circuit 13. The inverting output terminal of the RSflip-flop IC11 is not used, and thus it is not shown in the figure. Thedrive circuit 13 uses, as a power supply voltage, the voltage suppliedto the integrated circuit 12.

[0071] Of the above-described elements, the current source I, thecomparators, IC5, IC6, IC8, and IC9, the AND circuits IC7 and IC10, theRS flip-flop IC11, the drive circuit 13, the offset voltage source Voff,and the reference voltage source Vref define the integrated circuit 12.

[0072] The operation of the switching power supply 10 is described belowwith reference to FIG. 4. FIG. 4 shows changes, with time, in thevoltage Vbias of the feedback winding N3, the voltage Vc7 across thecapacitor C7 defining the first capacitor and also as the thirdcapacitor, and the voltage Vc5 across the second capacitor C6, under (a)the rated load condition, and (b) light load condition of the switchingpower supply 10. In FIG. 4, Vfb denotes the collector voltage of thephototransistor PT, which is applied as a feedback voltage to thenon-inverting input terminal of the comparator IC6 and also to theinverting input terminal of the comparator IC8. Vfb changes with load.However, when the output voltage does not change, Vfb is maintained at asubstantially constant value. Vref denotes the voltage of the referencevoltage source Vref, which is connected to the inverting input terminalof the comparator IC9.

[0073] The operation under the non-low load condition is described belowfor various operation stages.

[0074] (t=0 to t1)

[0075] When the switching device Q1 turns on, a current flows throughthe primary winding N1, and the voltage Vbias of the feedback winding N3becomes positive. As a result, charging of the capacitors C6 and C7begins. In the operation under the rated load condition, Vfb is at ahigh level, and thus, the voltage Vc7 across the capacitor C7 is lessthan Vfb. As a result, the output of the comparator IC6 is at theH-level and the output of the comparator IC8 is at the L-level.Furthermore, the output of the comparator IC5 is at the H-level becausethe voltage Vbias is applied to the non-inverting input terminalthereof.

[0076] (t=t1 to t2)

[0077] If the voltage Vc6 across the capacitor C6 increases and exceedsVref, the output of the comparator IC9 goes to the H-level. However, atthis point of time, the voltage Vc7 across the capacitor C7 does not yetexceed Vfb, and thus, the output of the comparator IC8 still remains atthe L-level and the output of the AND circuit IC10 also remains at theL-level. Herein, the output of the comparator IC6 is at the H-levelbecause the inputs thereof are connected in a manner that is opposite tothe comparator IC8.

[0078] (t=t2 to t3)

[0079] If the voltage Vc7 across the capacitor C7 exceeds Vfb, theoutput of the comparator IC8 goes to the H-level, and the two inputs ofthe AND circuit IC10 both become high. As a result, the output of theAND circuit IC10 goes to the H-level. Herein, the output level of thecomparator IC6 becomes low, and thus, the output level of the ANDcircuit IC7 becomes low. When the output level of the AND circuit 10becomes high, the rising edge of the output voltage serves as a triggerthat resets the RS flip-flop IC11. If the RS flip-flop IC11 is reset,the output thereof goes to the L-level, which causes the switchingdevice Q1 to be turned off via the drive circuit 13, and thus, theon-period terminates. That is, crossing the voltage Vfb determined bythe feedback signal in the change in the voltage Vc7 across thecapacitor C7 functioning as the first capacitor functions as a triggerthat determines the timing of the turning-off of the switching device Q1and thus, determines the on-period.

[0080] Because the resistance between the collector and the emitter ofthe phototransistor PT decreases as the load decreases and thus, as theintensity of light received from the light emitting diode PD increases,Vfb decreases with decreasing load. Therefore, the time needed for thevoltage Vc7 across the capacitor C7 to exceed Vfb decreases as the loaddecreases. Thus, a circuit including the resistor R8, the capacitor C7,the comparator IC8, the AND circuit IC10, the RS flip-flop IC11, thedrive circuit 13, and the feedback voltage generator functions as anon-period control circuit that controls the on-period of the switchingdevice Q1 such that the on-period decreases with decreasing load in thenon-light load state.

[0081] When the switching device Q1 turns off, a current starts to flowfrom the secondary winding N2 into the rectifying and smoothing circuit2, and the voltage Vbias of the feedback winding N3 becomes negative. Asa result, discharging of the capacitors C6 and C7 starts. If thedischarging of the capacitor C7 starts, the voltage Vc7 across thecapacitor C7 immediately becomes lower than Vfb, and thus, the output ofthe comparator IC8 goes to the L-level and the output of the AND circuitIC10 goes to the L-level. On the other hand, the output of thecomparator IC6 goes to the H-level, and the negative voltage Vbias isapplied to the non-inverting input terminal of the comparator IC5. As aresult, the output of the comparator IC5 goes to the L-level, and theoutput of the AND circuit IC7 goes to the L-level. If furtherdischarging of the capacitor C6 occurs, the voltage Vc6 across thecapacitor C6 becomes lower than Vref, and the output level of thecomparator IC9 becomes low.

[0082] As understood from the above description, because the voltage Vc7across the capacitor C7 quickly returns to a level lower than Vfb afterbecoming higher than Vfb, the turning-off of the switching device Q1occurs when the voltage Vc7 reaches Vfb without exceeding it.

[0083] (t=t3 to t4)

[0084] When the switching device Q1 is in the off-state, if the currentflowing out of the secondary winding N2 into the rectifying andsmoothing circuit 2 becomes zero, the voltage Vbias of the feedbackwinding N3 starts to resonate.

[0085] (t≧t4)

[0086] The first positive portion of a resonating wave of the voltageVbias causes the voltage of the non-inverting input terminal of thecomparator IC5 to become higher than the voltage of the inverting inputterminal, and thus, the output level of the comparator IC5 becomes high.Because the output level of the comparator IC6 is already high, theoutput of the AND circuit IC7 goes to the H-level. The rising edge ofthis transition of the output voltage serves as a trigger that sets theRS flip-flop IC11. If the RS flip-flop IC11 is set, the output thereofgoes to the H-level, which causes the switching device Q1 to be turnedon via the drive circuit 13. That is, the transition of the current fromthe secondary winding N2 to the rectifying and smoothing circuit 2 tozero serves as a trigger that turns on the switching device Q1. If theswitching device Q1 turns on, the resonance of the voltage Vbias stops,and the voltage Vbias again becomes positive as at t=0. Thereafter, theprocess after t=0 is repeated.

[0087] In the operation under the rated load condition (under thenon-low load condition), as described above, when the current flowingthrough the primary winding N1 stops, a current immediately starts toflow from the secondary winding N2 to the rectifying and smoothingcircuit 2, and, if the current flowing out of the secondary winding N2stops, the switching device Q1 immediately turns on and a current startsto flow through the primary winding N1. Thus, under the rated loadcondition, the switching power supply 10 operates in thecurrent-critical mode.

[0088] The operation under the low load condition is described below forvarious operation stages.

[0089] (t=0 to t1)

[0090] When the switching device Q1 turns on, a current flows throughthe primary winding N1, and the voltage Vbias of the feedback winding N3becomes positive. As a result, charging of the capacitors C6 and C7begins. At the point of time at which the switching device Q1 turns on,as will be described later, the capacitor C6 is in a completelydischarged state, and thus, the voltage Vc6 across the capacitor C6 isequal to 0 V. As a result of the change of the voltage Vbias to apositive value, the output level of the comparator IC5 becomes high. Inthe light load state, because Vfb is low and the voltage Vc7 across thecapacitor C7 is greater than Vfb, the output of the comparator IC6 is atthe L-level and the output of the comparator IC8 is at the H-level. Thatis, the increase in the voltage Vc7 of the capacitor C7 defining thefirst capacitor does not function as a trigger that causes the switchingdevice Q1 to turn off, and thus, the operation of the on-period controlcircuit is suppressed. Furthermore, because the voltage Vc6 across thecapacitor C6 is less than Vref, the output of the comparator IC9 is atthe L-level.

[0091] (t=t1 to t2)

[0092] When the voltage Vc6 across the capacitor C6 exceeds Vref, theoutput of the comparator IC9 goes to the H-level. Because the outputlevel of the comparator IC8 is already high, the output of the ANDcircuit IC10 goes to the H-level. On the other hand, the voltage Vc7across the capacitor C7 is greater than Vfb, and thus, the output of thecomparator IC6 remains at the L-level, and the output of the AND circuitIC7 remains at the L-level. When the output level of the AND circuit 10becomes high, the rising edge of the output voltage serves as a triggerthat resets the RS flip-flop IC11. If the RS flip-flop IC11 is reset,the output thereof goes to the L-level, which causes the switchingdevice Q1 to be turned off via the drive circuit 13. That is, in theincrease in the voltage Vc6 across the capacitor C6 defining the secondcapacitor, crossing the voltage Vref serves as a trigger that determinesthe timing of the turning-off the switching device Q1 and thus,determines the on-period. In other words, the minimum on-period of theswitching device Q1 is determined by a period required for the voltageVc6 across the capacitor C6 to exceed Vref after starting to increasefrom 0 V. Thus, a circuit including the resistor R6, the capacitor C6,the diode D2, the comparator IC9, and the reference voltage source Vrefserves as a minimum on-period setting circuit that disables theon-period control circuit from turning on the switching device in theoperation under the low load condition.

[0093] When the switching device Q1 turns off, a current starts to flowfrom the secondary winding N2 into the rectifying and smoothing circuit2, and the voltage Vbias of the feedback winding N3 becomes negative. Asa result, discharging of the capacitors C6 and C7 starts. If thedischarging of the capacitor C6 begins, the voltage Vc6 across thecapacitor C6 immediately becomes less than Vref, and thus, the output ofthe comparator IC9 goes to the L-level and the output of the AND circuitIC10 also goes to the L-level.

[0094] (t=t2 to t3)

[0095] When the switching device Q1 is in the off-state, if the currentflowing out of the secondary winding N2 into the rectifying andsmoothing circuit 2 becomes zero, the voltage Vbias of the feedbackwinding N3 starts to resonate. The first positive portion of theresonating wave of the voltage Vbias causes the voltage of thenon-inverting input terminal of the comparator IC5 to become greaterthan the voltage of the inverting input terminal, and thus, the outputlevel of the comparator IC5 becomes high. However, at this point intime, the voltage Vc7 across the capacitor C7 is still greater than Vfband the output of the comparator IC6 still remains at the L-level.Therefore, the output of the AND circuit IC7 remains at the L-level, andthus, the RS flip-flop IC11 is not set. That is, the turning-on of theswitching device Q1 by the resonating voltage Vbias is prevented.Therefore, after that, no current flows through the primary winding N1and the secondary winding N2. As a result, the current-critical mode isaborted. Thus, the resonating voltage Vbias attenuates with passage oftime, and the output level of the comparator IC5 becomes alternatelyhigh and low. When the voltage Vbias has completely attenuated, thevoltage of the non-inverting input terminal of the comparator IC5becomes equal to zero. In this state, because the inverting inputterminal of the comparator IC5 is connected to the offset voltage sourceVoff having a small negative value, the output level of the comparatorIC5 becomes high. On the other hand, the capacitor C7 is furtherdischarged, and the voltage Vc7 across the capacitor C7 furtherdecreases. The capacitor C6 is also further discharged, and the voltageVc6 across the capacitor C6 decreases with time toward 0 V.

[0096] (t≧t3)

[0097] If the voltage Vc7 across the capacitor C7 decreases to becomesless than Vfb, the output of the comparator IC6 goes to the H-level, andthe output of the comparator IC8 goes to the L-level. Because the outputlevel of the comparator IC5 is already high, the output of the ANDcircuit IC7 goes to the H-level. The rising edge of this transition ofthe output voltage serves as a trigger that sets the RS flip-flop IC11.If the RS flip-flop IC11 is set, the output thereof goes to the H-level,which causes the switching device Q1 to be turned on via the drivecircuit 13. Thus, the crossing of the voltage Vc7 of the capacitor C7also defining the third capacitor across Vfb serves as a trigger thatdetermines the timing of the turning-on of the switching device Q1.Because Vfb decreases with decreasing load, the time required for thevoltage Vc7 across the capacitor C7 to decrease until it becomes lessthan Vfb increases with decreasing load. Thus, a circuit including theresistor R8, the capacitor C7, the comparators IC5 and IC6, the offsetvoltage source Voff, the AND circuit IC7, the RS flip-flop IC11, thedrive circuit 13, and the feedback voltage generator defines anoff-period control circuit that controls the off-period of the switchingdevice Q1 such that the off-period increases with decreasing load in thelight load state. Note that the capacitance of the capacitor C6 and theresistance of the resistor R6 are set such that the capacitor C6 iscompletely discharged by the above-described point of time. Thereafter,the process after t=0 is repeated.

[0098] In the switching power supply 10, as described above withreference to FIGS. 3 and 4, the on-period control circuit controls theon-period of the switching device in the operation under the non-lowload condition such that the output voltage is maintained at a constantvalue, while, in the operation under the low load condition, the minimumon-period setting circuit sets the on-period of the switching device tothe minimum on-period, and the off-period control circuit controls theoff-period such that the output voltage is maintained at the constantvalue.

[0099] Thus, the increase in the switching frequency in the light loadstate is suppressed and the switching frequency with decreasing load isgreatly reduced, thereby achieving a reduction in switching loss in thelight-load state. Furthermore, because the off-period of the switchingdevice is controlled continuously depending on the load in thelight-load state, intermittent oscillation is prevented and outputripples are greatly decreased.

[0100] As described above, when the on-period control circuit isoperating under the non-low load condition, the voltage Vc7 across thecapacitor C7 defining the first capacitor increases from a low voltageand crosses the voltage Vfb determined by the feedback signal. On theother hand, when the minimum on-period setting circuit is operatingunder the low load condition, the voltage Vc7 across the capacitor V7crosses the voltage Vfb after it decreases from a high voltage. That is,when the on-period control circuit is enabled, the timing of theturning-off of the switching device is determined at the time at whichthe voltage across the first capacitor crosses the feedback voltage inone direction. On the other hand, when the on-period control circuit isdisabled by the minimum on-period setting circuit, the timing of theturning-on of the switching device is determined at the time at whichthe voltage across the first capacitor crosses the feedback voltage inthe opposite direction. As described above, because one capacitordefines both first and third capacitors, a reduction in the number ofcomponents is achieved. As a result, the size and cost of the switchingpower supply are greatly reduced.

[0101] By providing the majority of the components of the controlcircuit as an integrated circuit, the total number of components isgreatly reduced, and thus, the size and cost are further reduced.

[0102]FIG. 5 is a circuit diagram of a switching power supply accordingto still another preferred embodiment of the present invention. In FIG.5, similar components to those in FIG. 3 are denoted by similarreference numerals and the description thereof is omitted.

[0103] As shown in FIG. 5, the switching power supply 20 includes acontrol circuit 21 provided between a feedback winding N3 and the gateof a switching device Q1, wherein a main portion of the control circuit21 is defined by an integrated circuit 22. The control circuit 21including constituent elements of the integrated circuit 22 is describedbelow.

[0104] One end of the feedback winding N3 is connected to the integratedcircuit 22 via a rectifying and smoothing circuit including a diode D4and a capacitor C8. The voltage output from the rectifying and smoothingcircuit is applied to each element of the integrated circuit 22. Thenode between the diode D4 and the capacitor C8 is connected to a DCpower supply Vin via a starting resistor R1.

[0105] A phototransistor PT coupled with the light emitting diode PD inthe output voltage detecting circuit 3 is arranged such that thecollector is connected to a constant current source I the emitter isconnected to ground, a capacitor C3 is connected between the collectorand emitter, and the collector is connected to an inverting inputterminal of a comparator IC6 and also to a non-inverting input terminalof a comparator IC12 via a diode D5. The inverting input terminal of thecomparator IC12 is also connected to a reference voltage source Vref.The constant current source I is produced using the voltage supplied tothe integrated circuit 22 from the rectifying and smoothing circuitincluding the diode D4 and the capacitor C8.

[0106] The one end of the feedback winding N3 is connected to anon-inverting input terminal of a comparator IC5. An inverting inputterminal of the comparator IC5 is connected to an offset voltage sourceVoff having a small negative voltage such that the output of thecomparator IC5 becomes high when the voltage of the non-inverting inputterminal is equal to zero.

[0107] The one end of the feedback winding N3 is also connected toground via a resistor R8 and a capacitor C7, and the node between theresistor R8 and the capacitor C7 is connected to the inverting inputterminal of the comparator IC6 and also to the non-inverting inputterminal of the comparator IC12. The capacitor C7 defines the first,second, and third capacitors.

[0108] The outputs of the comparators IC5 and IC6 are connected to twoinputs of an AND circuit IC7, and the output of the AND circuit IC7 isconnected to a set terminal S of an RS flip-flop IC11. The output of thecomparator IC12 is connected to a reset terminal RS of the RS flip-flopIC11. The output terminal Q of the RS flip-flop IC11 is connected to thegate of the switching device Q1 via the drive circuit 13. The drivecircuit 13 uses, as a power supply voltage, the voltage supplied to theintegrated circuit 12.

[0109] The output of the driver circuit 13 is connected to the base of atransistor Q5 via a monostable multivibrator 23 and a resistor R9, andthe collector of the transistor Q5 is connected to an inverting inputterminal of a comparator IC6, and the emitter of the transistor Q5 isconnected to ground.

[0110] Of the above elements, the current source I, the comparators, thediode D5, the transistor Q5, the resistor R9, the monostablemultivibrator 23, the comparators IC5, IC6, and IC12, the AND circuitIC7, the RS flip-flop IC11, the drive circuit 13, the offset voltagesource Voff, and the reference voltage source Vref preferably define theintegrated circuit 22.

[0111] The operation of the switching power supply 20 is described belowwith reference to FIG. 6. FIG. 6 shows temporal changes in the voltageVbias of the feedback winding N3 and the voltage Vc7 across thecapacitor C7 defining the first capacitor, the second capacitor, and thethird capacitor in the switching power supply 20, wherein (a) showschanges in the operation under the rated load condition, and (b) showschanges in the operation under the low load condition. In FIG. 6, Vfbdenotes the collector voltage of the phototransistor PT, which isapplied as a feedback voltage to the non-inverting input terminal of thecomparator IC6 and also to the inverting input terminal of thecomparator IC12 via the diode D5. Vfb changes with load. However, whenthe output voltage is maintained at a constant value, for example, inthe operation with the rated load, Vfb is maintained at a substantiallyconstant value. Vref denotes the voltage of the reference voltage sourceVref, which is connected to the inverting input terminal of thecomparator IC12.

[0112] The operation under the rated-load condition is described belowfor various operation stages.

[0113] (t=0 to t1)

[0114] When the switching device Q1 turns on, a current flows throughthe primary winding N1, and the voltage Vbias of the feedback winding N3becomes positive. As a result, charging of the capacitor C7 starts.Under the rated load condition, Vfb is at a high level and the capacitorC7 has been reset just before the turning-on the switching device Q1, aswill be described later, the voltage Vc7 across the capacitor C7 is lessthan Vfb at this point of time and the output of the comparator IC6 isat the H-level. Vfb is further greater than Vref, and thus, Vfb isapplied to the inverting input terminal of the comparator IC12 via thediode D5. As a result, the output of the comparator IC12 is at theL-level. Furthermore, the output of the comparator IC5 is at the H-levelbecause the voltage Vbias is applied to the non-inverting input terminalthereof. The output of the AND circuit IC7 is also at the H-level.

[0115] (t=t1 to t2)

[0116] Even when the voltage Vc7 across the capacitor C7 exceeds Vref,the output of the comparator IC12 remains at the L-level because Vfbthat is greater than Vref is applied to the inverting input terminal ofthe comparator IC12.

[0117] (t=t2 to t3)

[0118] When the voltage Vc7 across the capacitor C7 exceeds Vfb, theoutput of the comparator IC12 goes to the H-level. On the other hand,the output level of the comparator IC6 becomes low, and thus, the outputlevel of the AND circuit IC7 becomes low. When the output level of thecomparator IC12 becomes high, the rising edge of the output voltageserves as a trigger that resets the RS flip-flop IC11. If the RSflip-flop IC11 is reset, the output thereof goes to the L-level, whichcauses the switching device Q1 to be turned off via the drive circuit13, and thus, the on-period terminates. That is, in the change in thevoltage Vc7 of the capacitor C7 defining the first capacitor, crossingthe voltage Vfb determined by the feedback signal functions as a triggerthat determines the timing of the turning-off of the switching device Q1and thus determines the on-period of the switching device Q1.

[0119] Because the resistance between the collector and the emitter ofthe phototransistor PT decreases as the load reduces and thus, as theintensity of light received from the light emitting diode PD increases,Vfb decreases with decreasing load. Therefore, the time needed for thevoltage Vc7 across the capacitor C7 to exceed Vfb decreases withreducing load. Thus, a circuit including the resistor R8, the capacitorC7, the comparator IC12, the diode D5, the RS flip-flop IC11, the drivecircuit 13, and the feedback voltage generator define an on-periodcontrol circuit that controls the on-period of the switching device Q1in the operation under the non-low load condition such that theon-period decreases with decreasing load.

[0120] When the switching device Q1 turns off, a current starts to flowfrom the secondary winding N2 into the rectifying and smoothing circuit2, and the voltage Vbias of the feedback winding N3 becomes negative. Asa result, discharging of the capacitor C7 starts. If the discharging ofthe capacitor C7 begins, the voltage Vc7 across the capacitor C7immediately becomes less than Vfb, and thus, the output of thecomparator IC12 goes to the L-level. On the other hand, the output ofthe comparator IC6 goes to the H-level, and the output of the comparatorIC5 goes to the L-level in response to the application of the negativevoltage Vbias to the non-inverting input terminal of the comparator IC5.As a result, the output of the AND circuit IC7 goes to the L-level.

[0121] As understood from the above-described description, because thevoltage Vc7 across the capacitor C7 quickly returns to a level less thanVfb after becoming higher than Vfb, the turning-off of the switchingdevice Q1 occurs when the voltage Vc7 reaches Vfb without exceeding it.

[0122] (t=t3 to t4)

[0123] When the switching device Q1 is in the off-state, if the currentflowing out of the secondary winding N2 into the rectifying andsmoothing circuit 2 becomes zero, the voltage Vbias of the feedbackwinding N3 starts to resonate.

[0124] (t≧t4)

[0125] The first positive portion of a resonating wave of the voltageVbias causes the voltage of the non-inverting input terminal of thecomparator IC5 to become greater than the voltage of the inverting inputterminal, and thus, the output level of the comparator IC5 becomes high.Because the output level of the comparator IC6 is already high, theoutput of the AND circuit IC7 goes to the H-level. The rising edge ofthis transition of the output voltage serves as a trigger that sets theRS flip-flop IC11. If the RS flip-flop IC11 is set, the output thereofgoes to the H-level, which causes the switching device Q1 to be turnedon via the drive circuit 13. That is, the transition-to-zero of thecurrent from the secondary winding N2 to the rectifying and smoothingcircuit 2 serves as a trigger that turns on the switching device Q1. Ifthe switching device Q1 turns on, the resonance of the voltage Vbiasstops, and the voltage Vbias again becomes positive.

[0126] The signal that is output from the drive circuit 13 to turn onthe switching device Q1 serves as a trigger signal to the monostablemultivibrator 23. This causes the output level of the monostablemultivibrator 23 to temporarily become high. This output is applied tothe base of the transistor Q5 via the resistor R9, and thus, thetransistor Q5 temporarily turns on. The transition of the transistor Q5into the on-state causes the charge stored in the capacitor C7 to beinstantly released, and thus, the voltage Vc7 across the capacitor C7 isreset to 0 V. Thereafter, the process after t=0 is repeated.

[0127] Under the rated load condition (under non-low load condition), asdescribed above, when the current flowing through the primary winding N1stops, a current immediately starts to flow from the secondary windingN2 to the rectifying and smoothing circuit 2, and, if the currentflowing out of the secondary winding N2 stops, the switching device Q1immediately turns on and a current starts to flow through the primarywinding N1. Thus, under the rated load condition, the switching powersupply 20 operates in the current-critical mode.

[0128] The operation under the low load condition is described below forvarious operation stages.

[0129] (t=0 to t1)

[0130] When the switching device Q1 turns on, a current flows throughthe primary winding N1, and the voltage Vbias of the feedback winding N3becomes positive. As a result, charging of the capacitor C7 begins. As aresult of the change of the voltage Vbias to a positive value, theoutput level of the comparator IC5 becomes high. In the operation underthe low load condition, Vfb becomes less than Vref, and thus, Vref isapplied to the inverting input terminal of the comparator IC12. Becausethe capacitor C7 is reset just before this point of time, as will bedescribed later, the voltage Vc7 across the capacitor C7 issubstantially equal to 0 V and thus lower than Vref. As a result, theoutput level of the comparator IC6 becomes high, and the output level ofthe comparator IC12 becomes low.

[0131] (t=t1 to t2)

[0132] If the voltage Vc7 across the capacitor C7 increases until itbecomes greater than Vfb, the output level of the comparator IC6 becomeslow. However, the voltage Vref applied to the inverting input terminalof the comparator IC12 is still greater than the voltage Vc7, the outputof the comparator IC12 remains at the L-level. This means that thecrossing of Vfb in the increase in the voltage Vc7 across the capacitorC7 defining the first capacitor does not function as a trigger thatcauses the switching device Q1 to turn off. That is, the on-periodcontrol circuit is disabled.

[0133] (t=t2 to t3)

[0134] When the voltage Vc7 across the capacitor C7 exceeds Vref, theoutput level of the comparator IC12 becomes high, and the rising edge ofthis transition of the output voltage serves as a trigger that resetsthe RS flip-flop IC11. If the RS flip-flop IC11 is reset, the outputthereof goes to the L-level, which causes the switching device Q1 to beturned off via the drive circuit 13. Thus, in the increase in thevoltage Vc7 of the capacitor C7 also defining the second capacitor,crossing the voltage Vref functions as a trigger that determines thetiming of the turning-off of the switching device Q1 and thus,determines the on-period. In other words, the minimum on-period of theswitching device Q1 is determined by a period needed for the voltage Vc7across the capacitor C7 to increase until it exceeds Vref. Thus, acircuit including the resistor R8, the capacitor C7, the comparatorIC12, and the reference voltage source Vref defines a minimum on-periodsetting circuit that disables the on-period control circuit from turningon the switching device in the operation under the low load condition.The reason why the minimum on-period is given by the period needed forthe voltage Vc7 across the capacitor C7 to exceed Vref will be describedlater. The reason why the minimum on-period setting circuit includes, inaddition to the above-described elements, the monostable multivibrator23, the resistor R9, and the transistor Q5 will also be described later.

[0135] When the switching device Q1 turns off, a current starts to flowfrom the secondary winding N2 into the rectifying and smoothing circuit2, and the voltage Vbias of the feedback winding N3 becomes negative. Asa result, discharging of the capacitor C7 begins. If the discharging ofthe capacitor C7 starts, the voltage Vc7 across the capacitor C7immediately becomes less than Vfb, and thus, the output level of thecomparator IC12 becomes low.

[0136] (t=t3 to t4)

[0137] When the switching device Q1 is in the off-state, if the currentflowing out of the secondary winding N2 into the rectifying andsmoothing circuit 2 becomes zero, the voltage Vbias of the feedbackwinding N3 starts to resonate. The first positive portion of theresonating wave of the voltage Vbias causes the voltage of thenon-inverting input terminal of the comparator IC5 to become greaterthan the voltage of the inverting input terminal, and thus, the outputlevel of the comparator IC5 becomes high. However, at this point intime, the voltage Vc7 across the capacitor C7 is still greater than Vfband the output of the comparator IC6 still remains at the L-level.Therefore, the output of the AND circuit IC7 remains at the L-level, andthus, the RS flip-flop IC11 is not set. That is, the turning-on of theswitching device Q1 by the resonating voltage Vbias is prevented.Therefore, after that, no current flows through the primary winding N1and the secondary winding N2. As a result, the current-critical mode isaborted. Thus, the resonating voltage Vbias attenuates with passage oftime, and the output level of the comparator IC5 becomes alternatelyhigh and low. When the voltage Vbias has completely attenuated, thevoltage of the non-inverting input terminal of the comparator IC5becomes equal to zero. In this state, because the inverting inputterminal of the comparator IC5 is connected to the offset voltage sourceVoff having a small negative value, the output level of the comparatorIC5 becomes high. On the other hand, the capacitor C7 is furtherdischarged, and the voltage Vc7 across the capacitor C7 furtherdecreases.

[0138] (t≧t4)

[0139] If the voltage Vc7 across the capacitor C7 decreases until itbecomes less than Vfb, the output of the comparator IC6 goes to theH-level. Because the output level of the comparator IC5 is already high,the output of the AND circuit IC7 goes to the H-level. The rising edgeof this transition of the output voltage serves as a trigger that setsthe RS flip-flop IC11. If the RS flip-flop IC11 is set, the outputthereof goes to the H-level, which causes the switching device Q1 to beturned on via the drive circuit 13. Thus, the crossing of the voltageVc7 of the capacitor C7 also defining the third capacitor across Vfbserves as a trigger that determines the timing of the turning-on of theswitching device Q1. Because Vfb decreases with decreasing load, thetime needed for the voltage Vc7 across the capacitor C7 to decreaseuntil it becomes less than Vfb increases with decreasing load. Thus, acircuit including the resistor R8, the capacitor C7, the comparators IC5and IC6, the offset voltage source Voff, the AND circuit IC7, the RSflip-flop IC11, the drive circuit 13, and the feedback voltage generatordefines an off-period control circuit that controls the off-period ofthe switching device Q1 such that the off-period increases withdecreasing load in the light load state.

[0140] The signal that is output from the drive circuit 13 to turn onthe switching device Q1 is also applied as a trigger signal to themonostable multivibrator 23. This causes the output level of themonostable multivibrator 23 to temporarily become high. This output isapplied to the base of the transistor Q5 via the resistor R9, and thus,the transistor Q5 temporarily turns on. The transition of the transistorQ5 into the on-state causes the charge stored in the capacitor C7 to bereleased in an instant, and thus, the voltage Vc7 across the capacitorC7 is reset to 0 V. Thereafter, the process after t=0 is repeated.

[0141] Now, the reason why the minimum on-period is given by the periodneeded for the voltage Vc7 across the capacitor C7 to exceed Vref, andthe reason why the minimum on-period setting circuit includes themonostable multivibrator 23, the resistor R9, and the transistor Q5 aredescribed. If the monostable multivibrator 23 is not included, thecharging of the capacitor C7 after the switching device Q1 turns onstarts from a state in which the capacitor C7 has been charged until thevoltage Vc7 across the capacitor C7 becomes equal to Vfb. Vfb changesdepending on the load, although Vref is constant. Therefore, the periodneeded for the voltage Vc7 across the capacitor C7 to be charged to Vrefchanges depending on the load. This period corresponds to the on-periodin the operation under the low load condition. Therefore, if themonostable multivibrator 23 is not included, the on-period varies in theoperation under the low load condition, the minimum on-period cannot beset. In contrast, if the capacitor C7 is reset by the monostablemultivibrator 23 when the switching device Q1 turns on, the capacitor C7is always charged starting from 0 V until the voltage becomes equal toVref, and thus, the charging time becomes constant. This makes itpossible to set the minimum on-period regardless of the magnitude of theload. For the above-described reason, the minimum on-period settingcircuit includes the monostable multivibrator 23, the resistor R9, andthe transistor Q5.

[0142] In the switching power supply 20, as described above withreference to FIGS. 5 and 6, the on-period control circuit controls theon-period of the switching device in the operation under the non-lowload condition such that the output voltage is maintained at a constantvalue, while, in the operation under the low load condition, the minimumon-period setting circuit sets the on-period of the switching device tothe minimum on-period, and the off-period control circuit controls theoff-period such that the output voltage is maintained at the constantvalue.

[0143] Thus, the increase in the switching frequency in the operationunder the low load condition is suppressed and the switching frequencywith decreasing load is decreased, thereby greatly reducing switchingloss in the light-load state. Furthermore, because the off-period of theswitching device is controlled continuously depending on the load in thelight-load state, intermittent oscillation is prevented and outputripples are greatly reduced.

[0144] As described above, when the on-period control circuit isoperating under the non-low load condition, the voltage Vc7 across thecapacitor C7 defining the first capacitor increases from a low voltageand crosses the voltage Vfb determined by the feedback signal. On theother hand, when the minimum on-period setting circuit is operatingunder the low load condition, the voltage Vc7 across the capacitor V7crosses the voltage Vfb after it decreases from a high voltage. That is,when the on-period control circuit is operating, the timing of theturning-off of the switching device is determined at the time at whichthe voltage across the first capacitor crosses the feedback voltage inone direction. On the other hand, when the on-period control circuit isdisabled by the minimum on-period setting circuit, the timing of theturning-on of the switching device is determined at the time at whichthe voltage across the first capacitor crosses the feedback voltage inthe opposite direction. As described above, because one capacitordefines both first and third capacitors, the number of components isreduced. As a result, the size and cost of the switching power supplyare greatly reduced.

[0145] Furthermore, because the functions of the first capacitor, thesecond capacitor, and the third capacitor are all performed by onecapacitor C7, the number of externally provided components is furtherreduced. Thus, the size and cost of the switching power supply isfurther reduced.

[0146] In each preferred embodiment described above, the minimumon-period setting circuit disables the on-period control circuit fromturning on the switching device in the operation under the low loadcondition. In this sense, the minimum on-period setting circuit isincluded in the on-period control circuit and which, in the operationunder the low load condition, disables the on-period control circuitfrom turning off the switching device for a fixed period after theswitching device turns on.

[0147] Furthermore, in each preferred embodiment described above, theturning-on or turning-off of the switching device is triggered when thefirst, second, and the third capacitors are charged or discharged untilthe voltages across them cross the reference voltage or the feedbackvoltage. However, the manners of triggering are not limited to thosedescribed above with reference to the preferred embodiments. Forexample, the feedback voltage may be set such that it increases withreducing load, and the manner in which the capacitor voltage exceeds thefeedback voltage may be modified such that exceeding occurs in thedischarging process instead of the charging process or such thatexceeding occurs in the charging process instead of the dischargingprocess.

[0148] Furthermore, in each preferred embodiment described above, theturning-on or turning-off of the switching device is triggered when thevoltage across the first, second, or third capacitor crosses thereference voltage or the feedback voltage. However, in a practicalsense, the turning-on or turning-off of the switching device istriggered when the capacitor voltage reaches the reference voltage orthe feedback voltage. That is, the voltage across the first, second, orthird capacitor need not cross the reference voltage or the feedbackvoltage. Therefore, the comparator may be replaced with a circuit thatoutputs a trigger signal when two inputs become equal to each other suchthat the turning-on or turning-off of the switching device is triggeredwhen the voltage across the firsts second, or third capacitor reachesthe reference voltage or the feedback voltage.

[0149]FIG. 7 is a perspective view of an electronic apparatus accordingto another preferred embodiment of the present invention. In the exampleshown in FIG. 7, a printer 30 is described as the electronic apparatuswherein the switching power supply 1 according to other preferredembodiments of the present invention is provided in a power supplycircuit of the printer 30.

[0150] In the operation of the printer 30, electric power is consumedwhen printing is performed. However, in a standby state in which aprinting operation is not performed, the load becomes low andsubstantially no electric power is consumed. Thus, use of the switchingpower supply 1 according to preferred embodiments of the presentinvention reduces the power loss in the standby state in which the loadis low, and thus, greatly improves the efficiency of the printer.

[0151] In the printer 30 shown in FIG. 7, the switching power supply 1shown in FIG. 1 is provided. Alternatively, the switching power supply10 or 20 shown in FIG. 3 or 5 may be provided to achieve similarfunctions and advantages.

[0152] The electronic apparatus according to the preferred embodiment ofthe present invention is not limited to the printer. The presentinvention may be applied to various types of electric apparatuses, suchas a notebook personal computer and a portable information device, whichneed a DC power supply capable of outputting a stabilized voltage.

[0153] In the switching power supply according to the preferredembodiments of the present invention, the output voltage is controlledat a constant value in accordance with the feedback signal by, in theoperation under the non-low load condition, controlling the on-periodwithin the range greater than the predetermined minimum on-period,while, in the operation under the low load condition, fixing theon-period to the minimum on-period and controlling the off-period,thereby achieving a reduction in the switching loss in the operationunder the low load condition. In this switching power supply accordingto preferred embodiments of the present invention, no intermittentoscillation occurs during the operation under the low load condition,and thus, the ripple caused by the intermittent oscillation isprevented.

[0154] In the electronic apparatus according to the preferred embodimentof the present invention including the switching power supply accordingto preferred embodiments of the present invention greatly improvedefficiency in the standby state is achieved.

[0155] While preferred embodiments of the invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the invention. The scope of the invention, therefore, is to bedetermined solely by the following claims.

What is claimed is:
 1. A switching power supply comprising: atransformer including a primary winding, a secondary winding and afeedback winding; a switching device connected in series to the primarywinding and having a control terminal; a control circuit connectedbetween the feedback winding and the control terminal of the switchingdevice; a rectifying and smoothing circuit connected to the secondarywinding, for rectifying and smoothing a voltage generated in thesecondary winding and outputting a resultant rectified and smoothedvoltage; and an output voltage detecting circuit for detecting thevoltage output from the rectifying and smoothing circuit and outputtinga feedback signal to the control circuit; wherein the control circuitcontrols the output voltage at a constant value in accordance with thefeedback signal by, in an operation under a non-low load condition,controlling the on-period within a range greater than a predeterminedminimum on-period, while, in an operation under a low load condition,fixing the on-period to the minimum on-period and controlling theoff-period.
 2. A switching power supply according to claim 1, whereinthe control circuit includes: an on-period control circuit that controlsan on-period of the switching device during an operation under a non-lowload condition such that the on-period decreases with decreasing load; aminimum on-period setting circuit that disables the on-period controlcircuit from turning on the switching device during an operation under alow load condition so that the on-period of the switching device doesnot become shorter than a predetermined minimum on-period; and anoff-period control circuit that controls an off-period of the switchingdevice during the operation under the low load condition such that, whenthe operation of the on-period control circuit is disabled by theminimum on-period setting circuit, the off-period increases withdecreasing load; whereby the output voltage is controlled at a constantvalue in accordance with the feedback signal.
 3. A switching powersupply according to claim 1, wherein during the operation under thenon-low load condition, the switching power supply operates in acurrent-critical mode.
 4. A switching power supply according to claim 2,wherein the on-period control circuit includes a first capacitor that ischarged or discharged during the on-period of the switching device, andthe timing of turning-off of the switching device is determined by atime at which the voltage across the first capacitor reaches a voltagedetermined by the feedback signal; the minimum on-period setting circuitincludes a second capacitor that is charged or discharged during theon-period of the switching device, the turning-off of the switchingdevice by the on-period control circuit is disabled until the voltageacross the second capacitor reaches a reference voltage; and theoff-period control circuit includes a third capacitor that is charged ordischarged during the off-period of the switching device, and the timingof turning-on of the switching device is determined by a time at whichthe voltage across the third capacitor reaches a voltage determined bythe feedback signal.
 5. A switching power supply according to claim 4,wherein the first capacitor also functions as the third capacitor.
 6. Aswitching power supply according to claim 4, wherein the first capacitoralso functions as the second and third capacitors.
 7. A switching powersupply according to claim 5, wherein when the on-period control circuitis operating, the timing of turning-off of the switching device isdetermined at a time at which the voltage across the first capacitorcrosses, in first direction, the voltage determined by the feedbacksignal; and when the operation of the on-period control circuit isdisabled by the minimum on-period setting circuit, the timing ofturning-on of the switching device is determined at a time at which thevoltage across the first capacitor crosses, in a second direction thatis opposite to the first direction, the voltage determined by thefeedback signal.
 8. A switching power supply according to claim 4,wherein the minimum on-period setting circuit is a device, included inthe on-period control circuit, for, during the operation under low loadcondition, preventing the on-period control circuit from turning off theswitching device for a fixed period after the switching device turns on.9. A switching power supply according to claim 8, wherein the minimumon-period setting circuit discharges the first capacitor when theswitching device turns on, and the minimum on-period setting circuitprevents the switching device from turning off thereafter until thevoltage of the first capacitor is charged thereafter until it reaches apredetermined voltage.
 10. An electronic apparatus comprising aswitching power supply according to claim
 1. 11. A switching powersupply comprising: a transformer having a primary winding, a secondarywinding and a feedback winding; a switching device connected in seriesto the primary winding and having a control terminal; a control circuitconnected between the feedback winding and the control terminal of theswitching device; a rectifying and smoothing circuit connected to thesecondary winding, for rectifying and smoothing a voltage generated inthe secondary winding and outputting a resultant rectified and smoothedvoltage; and an output voltage detecting circuit for detecting thevoltage output from the rectifying and smoothing circuit and outputtinga feedback signal to the control circuit; wherein the control circuitincludes: an on-period control circuit that controls an on-period of theswitching device during an operation under a non-low load condition suchthat the on-period decreases with decreasing load; a minimum on-periodsetting circuit that disables the on-period control circuit from turningon the switching device during an operation under a low load conditionso that the on-period of the switching device does not become shorterthan a predetermined minimum on-period; and an off-period controlcircuit that controls an off-period of the switching device during theoperation under the low load condition such that, when the operation ofthe on-period control circuit is disabled by the minimum on-periodsetting circuit, the off-period increases with decreasing load; wherebythe output voltage is controlled at a constant value in accordance withthe feedback signal.
 12. A switching power supply according to claim 11,wherein during the operation under the non-low load condition, theswitching power supply operates in a current-critical mode.
 13. Aswitching power supply according to claim 11, wherein the on-periodcontrol circuit includes a first capacitor that is charged or dischargedduring the on-period of the switching device, and the timing ofturning-off of the switching device is determined by a time at which thevoltage across the first capacitor reaches a voltage determined by thefeedback signal; the minimum on-period setting circuit includes a secondcapacitor that is charged or discharged during the on-period of theswitching device, the turning-off of the switching device by theon-period control circuit is disabled until the voltage across thesecond capacitor reaches a reference voltage; and the off-period controlcircuit includes a third capacitor that is charged or discharged duringthe off-period of the switching device, and the timing of turning-on ofthe switching device is determined by a time at which the voltage acrossthe third capacitor reaches a voltage determined by the feedback signal.14. A switching power supply according to claim 13, wherein the firstcapacitor also functions as the third capacitor.
 15. A switching powersupply according to claim 13, wherein the first capacitor also functionsas the second and third capacitors.
 16. A switching power supplyaccording to claim 14, wherein when the on-period control circuit isoperating, the timing of turning-off of the switching device isdetermined at a time at which the voltage across the first capacitorcrosses, in a first direction, the voltage determined by the feedbacksignal; and when the operation of the on-period control circuit isdisabled by the minimum on-period setting circuit, the timing ofturning-on of the switching device is determined at a time at which thevoltage across the first capacitor crosses, in a second direction thatis opposite to the first direction, the voltage determined by thefeedback signal.
 17. A switching power supply according to claim 13,wherein the minimum on-period setting circuit is a device, included inthe on-period control circuit, for, during the operation under low loadcondition, preventing the on-period control circuit from turning off theswitching device for a fixed period after the switching device turns on.18. A switching power supply according to claim 17, wherein the minimumon-period setting circuit discharges the first capacitor when theswitching device turns on, and the minimum on-period setting circuitprevents the switching device from turning off thereafter until thevoltage of the first capacitor is charged thereafter until it reaches apredetermined voltage.
 19. An electronic apparatus comprising aswitching power supply according to claim 11.